1. Field of the Invention
The present invention relates to a semiconductor material, e.g., containing silicon as the major component. More particularly, the present invention relates to a thin film transistor improved in properties and a process for fabricating the same. The semiconductor material according to the present invention enables fabrication of thin film semiconductor devices such as thin film transistors having excellent device characteristics.
2. Description of the Prior Art
Non-crystalline semiconductor materials (the so-called amorphous semiconductors) and polycrystalline semiconductor materials have been heretofore used for the fabrication of thin film semiconductor devices such as thin film field effect transistors and the like. The term “amorphous materials” as referred herein signifies not only the materials having a strict structural disordering in the atomic level, but also includes those having a short range ordering for a distance of about several nanometers. More concretely, “amorphous materials” include silicon materials having an electron mobility of 10 cm2/V·s or lower and materials having a carrier mobility lowered to 1% or less of the intrinsic carrier mobility of the corresponding semiconductor material. Accordingly, materials consisting of fine crystal aggregates which are composed of fine crystals about 10 nm in size, i.e., the materials known as microcrystals (having a grain diameter of 50 to 500 Å as calculated according to shira equation in Raman shift) or semi-amorphous materials (having lattice distortion therein and a peak in Raman shift at less than 521 cm−1, and having a structure comprising amorphous structure and crystalline structure with undefined boundary), are collectively referred to hereinafter as amorphous materials.
The use of an amorphous semiconductor such as amorphous silicon (a-Si) and amorphous germanium (a-Ge) in the fabrication of a semiconductor device is advantageous in that the process can be conducted at a relatively low temperature of 400° C. or even lower. Thus, much attention is paid now to a process using an amorphous material, because such a process is regarded as a promising one for the fabrication of liquid crystal displays and the like, to which a high temperature process cannot be applied.
However, a pure amorphous semiconductor has an extremely low carrier mobility (electron mobility and hole mobility). Thus, pure amorphous semiconductors are rarely applied as they are, for example, to channel-forming areas of thin film transistors (TFTs); in general, the pure amorphous semiconductor materials are subjected to the irradiation of a high energy beam such as a laser beam or a light emitted from a Xenon lamp, so that they may be once molten to recrystallize and thereby modified into a crystalline semiconductor material having an improved carrier mobility. Such a treatment of high energy beam irradiation is referred hereinafter collectively as “laser annealing”. It should be noted, however, that the high energy beam not necessary be a laser beam, and included in the high intensity beam is, for example, a powerful light emitted from a flash lamp which has a similar effect on the semiconductor material as the laser beam irradiation.
Generally, however, the semiconductor materials heretofore obtained by laser annealing are still low in the carrier mobility as compared with those of single crystal semiconductor materials. In the case of a silicon film, for example, the highest reported electron mobility is 200 cm2/V·s at best, which is a mere one seventh of the electron mobility of a single silicon, 1350 cm2/V·s. Moreover, the semiconductor characteristics (mainly mobility) of the semiconductor material thus obtained by the laser annealing process suffers poor reproducibility and also scattering of the mobility values over the single film. Those disadvantages lead to a low product yield of semiconductor devices having a plurality of elements fabricated on a single plane.